The present invention relates to a liquid crystal display device for displaying an image by using a liquid crystal material having a spontaneous polarization and on/off driving switching elements.
Along with the recent development of so-called information-oriented society, electronic apparatuses, such as personal computers and PDA (Personal Digital Assistants), have been widely used. Further, with the spread of such electronic apparatuses, portable apparatuses that can be used in offices as well as outdoors have been used, and there are demands for reduction in the size and weight of these apparatuses. Liquid crystal display devices have been widely used as one of the means to satisfy such demands. Liquid crystal display devices not only achieve reduction in their size and weight, but also include an indispensable technique in an attempt to achieve low power consumption in portable electronic apparatuses that are driven by batteries.
The liquid crystal display devices are mainly classified into the reflection type and the transmission type. In the reflection type liquid crystal display devices, light rays incident from the front face of a liquid crystal panel are reflected by the rear face of the liquid crystal panel, and an image is visualized by the reflected light; whereas in the transmission type liquid crystal display devices, the image is visualized by the transmitted light from a light source (back-light) placed on the rear face of the liquid crystal panel. Since the reflection type liquid crystal display devices have poor visibility because the reflected light amount varies depending on environmental conditions, transmission type liquid crystal display devices are generally used as display devices of, particularly, personal computers that display multi-color or full-color images.
Besides, the current color liquid crystal display devices are generally classified into the STN (Super Twisted Nematic) type and the TFT-TN (Thin Film Transistor-Twisted Nematic) type, based on the liquid crystal materials to be used. The STN type liquid crystal display devices can be produced at comparatively low costs, but they are not suitable for the display of a moving image because they are susceptible to crosstalk and comparatively slow in the speed of response. In contrast, the TFT-TN type liquid crystal display devices have better display quality than the STN type, but they require a back-light with high intensity because the light transmittance of the liquid crystal panel is only 4% or so at present. For this reason, in the TFT-TN type liquid crystal display devices, a lot of power is consumed by the back-light, and there would be a problem when used with a portable battery power source. Moreover, since a color display is realized using a color filter, a single pixel needs to be composed of three sub-pixels, and there are problems that it is difficult to provide a high-resolution display and the purity of the displayed colors is not sufficient.
In order to solve such problems, the present inventor et al. developed a field-sequential type liquid crystal display device. Since this field-sequential type liquid crystal display device does not require sub-pixels, it is possible to realize a higher-resolution display easily compared to color-filter type liquid crystal display devices. Moreover, since this device can use the color of light emitted by the light source as it is for display, without using a color filter, the displayed color has excellent purity. Furthermore, since the light unitization efficiency is high, this device has the advantage of low power consumption. However, in order to realize the field-sequential type liquid crystal display device, a high-speed response of liquid crystal is essential. Therefore, in order to realize a field-sequential liquid crystal display device having a significant advantage as mentioned above or achieve a high-speed response of a color-filter type liquid crystal display device, the present inventor et al. are conducting research and development on driving of liquid crystals such as a ferroelectric liquid crystal having a spontaneous polarization, which may achieve a 100 to 1000 times faster response compared to conventional driving, with a switching element such as a TFT (Thin Film Transistor).
In the ferroelectric liquid crystal, as shown in FIG. 1, the long-axis direction of the liquid crystal molecule changes by only a tilt angle θ with the application of a voltage. A liquid crystal panel sandwiching the ferroelectric liquid crystal therein is sandwiched by two polarizers whose polarization axes are orthogonal to each other, and the intensity of the transmitted light is changed using the birefringence caused by the change in the long-axis direction of the liquid crystal molecule. When the ferroelectric liquid crystal is driven by a switching element such as a TFT, the spontaneous polarization is switched according to the amount of charge injected (stored) in a pixel through the switching element, and the intensity of transmitted light changes.
A conventional ferroelectric liquid crystal has a memory characteristic. When the conventional ferroelectric liquid crystal is driven by a switching element such as a TFT, it utilizes the memory characteristic in a dark state (applied voltage: substantially 0 V), and obtains a light transmittance according to the applied voltage in a bright state. However, in the dark state utilizing the memory characteristic, a failure in writing, etc. deteriorates the memory characteristic and lowers the contrast ratio.
In order to solve this problem, TFT driving using a monostable ferroelectric liquid crystal has been tested. By causing the monostable state to be a dark state by using the monostable ferroelectric liquid crystal, an increase in the brightness in the dark state due to deterioration of the memory characteristic, which was observed in a bistable ferroelectric liquid crystal, is improved, but there is a problem of low light transmittance.
By the way, in a conventional liquid crystal display in which a liquid crystal such as a ferroelectric liquid crystal having a spontaneous polarization is driven by a switching element such as a TFT, if the size of spontaneous polarization per unit area is Ps and the electrode area of each pixel is A, 2Ps·A (the total charge amount of a switching current caused by complete reversal of the spontaneous polarization) is limited not to exceed a charge amount Q injected into each pixel through the switching element. In other words, the liquid crystal material, pixel electrodes, TFTs, etc. are designed so as to satisfy the condition of 2Ps·A≦Q.
However, with the application of a voltage of not higher than 7 V, since the magnitude Ps of spontaneous polarization satisfying the above-mentioned condition becomes as small as or less than 8 nC/cm2, Ps can not be increased much, and consequently the response is slow. Therefore, in the aspect of the response, particularly the response at low temperature, there is demand for an increase in the magnitude of spontaneous polarization. Moreover, there is a problem that the degree of freedom in selecting a liquid crystal material is low. When a liquid crystal material having a large spontaneous polarization is used in view of the response and selectable liquid crystal materials, it is necessary to increase Q, there is a problem of an increase in the applied voltage. In addition, as shown in FIG. 2, near the end of switching of the spontaneous polarization, since the change of the optical axis caused by switching of the liquid crystal is small, the ratio of a change in the intensity of the transmitted light due to an increase in the applied voltage is smaller, and a higher applied voltage is necessary to obtain a maximum transmitted light intensity.